Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 31	$ 25	$ 62	$ 49
Interest cost	50	50	100	101
Expected return on plan assets	(49)	(44)	(98)	(89)
Amortization of unrecognized (gain) loss and prior service cost (credit)	22	2	44	5
Settlement loss			1
Total	54	33	109	66
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	4	10	9
Interest cost	21	22	41	44
Expected return on plan assets	(1)		(1)	(1)
Amortization of unrecognized (gain) loss and prior service cost (credit)	(14)	(20)	(27)	(39)
Total	$ 11	$ 6	$ 23	$ 13